(a) Field of the Invention
The present invention relates to a unified polarizing plate that simultaneously functions as a negative C-plate and a polarizing plate, and a method for preparing the same.
(b) Description of the Related Art
The present invention relates to a polarizing plate comprising a transparent protection layer, a polarizing film and a transparent protection layer, in which at least one of the transparent protection layer has negative refractive index toward thickness direction. More particularly, the present invention relates to a unified polarizing plate capable of simultaneously functioning as a negative C-plate and as a polarizing plate, and a method for preparing the same.
Recently, liquid crystal display shows a tendency to increase its area from small size mobile phone, notebook monitor to middle to large size display device of computer monitor and television. Particularly, in the case of middle to large size liquid crystal display, it is important to have clear definition at wide view angle and improve luminosity contrast at ON/OFF of the operation cell, for securing quality of competitive display.
For such reasons, displays of various liquid crystal cell modes, such as Dual Domain TN, ASM (Axially symmetric aligned microcell), VA (vertical alignment), SE (surrounding electrode), PVA (Patterned VA), IPS (In-Plane Switching) mode, and the like, are under development. These modes respectively have unique liquid crystal alignment and have unique optical anisotropy. Thus, in order to compensate change in optical axis of linearly polarized light due to optical anisotropy of these liquid crystal modes, compensation films of various optical anisotropies are required.
In order for optical compensation of liquid crystal displays of various modes, it is important to develop an optical film that can control optical anisotropy precisely and effectively. An optical anisotropy is divided into Re, an in-plane phase difference value, and Rth, a phase difference value of in-plane fast axis (y-axis) and toward thickness direction (z-axis), as shown in the following equations 1 and 2.Rth=Δ(ny−nz)×d  [Equation 1]Re=Δ(nx−ny)×d  [Equation 2]                wherein, nx is refractive index of in-plane slow axis (x-axis), ny is refractive index of in-plane fast axis (y-axis), and nz is refractive index toward thickness direction (z-axis), and d is thickness of a film.        
In case one of Re or Rth calculated by the above equations are much larger than the other, the film can be used as a compensation film having uniaxial optical anisotropy, and in case both are larger than 0 and similar to each other, the film can be used as a compensation film having biaxial optical anisotropy.
As the compensation film having uniaxial optical anisotropy, A-plate (nx≠ny≅nz) and C-plate (nx≅ny≠nz) can be exemplified. Considering compensation of optical axis polarized due to liquid crystal only, an ideal compensation film should have an optical axis that is mirror image of the optical axis of liquid crystal layer, and thus, in the case of liquid crystal display of VA mode or TN mode oriented such that refractive index toward thickness direction is larger than that toward in-plane direction, a negative C-plate having negative birefringence toward thickness direction is required.
Since the negative C-plate has very small Re value, Rth can be calculated from the following equation 3 by measuring Rθ, which is the product of the length of light progress route and refractive index difference (ny−nθ) (wherein ny is refractive index of fast axis and θ is an angle between normal to the surface of a film and incident light).
                              R          th                =                                            R              θ                        ×            cos            ⁢                                                  ⁢                          θ              f                                                          sin              2                        ⁢                                                  ⁢                          θ              f                                                          [                  Equation          ⁢                                          ⁢          3                ]                            wherein, θf is internal angle.        
As a polymer material that can be used as such negative C-plate, discotic liquid crystal (U.S. Pat. No. 5,583,679) and polyimide having planar phenyl group in the main chain (U.S. Pat. No. 5,344,916), etc have been reported.
Since these materials have too large birefringence toward thickness direction and show light absorption at a visible ray, they cannot realize thickness of 30 to 150 μm that is suitable for a protection layer of a polarizing film. Thus, in case these materials are used for a protection layer of a polarizing film, precision coating thereof on a transparent protection layer is needed, which increases cost during coating process and causes non-uniformity of large phase difference even by slight difference in coating thickness due to comparatively large birefringence. Additionally, there is a problem of optical defect due to foreign substance such as dust remaining on the surface of coating film or existing in coating solution. Also, since materials comprising these aromatic compounds have large phase difference change rate according to wavelength, in case they are used for a protection film, compensation for wavelength dispersion due to them should also be considered.
Conventional polarizing plate consists of a polarizing film made of polyvinyl alcohol (PVA) and triacetate cellulose (TAC) protection layer that protects the polarizing film on both sides of the PVA polarizing film. Such polarizing plate is prepared by coating organic material such as discotic liquid crystal having negative birefringence toward thickness direction on a protection layer of the polarizing plate, or by laminating one or more films having slight birefringence toward thickness direction on a protection layer using an adhesive, in order for optical compensation toward thickness direction. Thus, its production process is complicated and is not favorable in terms of economy.
Additionally, although conventional TAC (triacetate cellulose) protection layer of polarizing plate has superior protection performance, it shows comparatively high moisture absorption, and thus there are problems of deterioration in polarizing degree and light leakage under high temperature high humidity conditions, and inferior durability.
Meanwhile, cyclic olefin copolymers are well known through literatures, and they have low dielectric constant due to high hydrocarbon content, and low hygroscopic property, are amorphous, and do not have light absorption at visible ray area due to π-conjugation, and thus have excellent light permeability. The cyclic monomers can be polymerized by ROMP (ring opening metathesis polymerization), HROMP (ring opening metathesis polymerization followed by hydrogenation), copolymerization with ethylene and homopolymerization, and the like, as shown in the following Reaction Formula 1. Wherein, in case the same kinds of monomers are polymerized using different polymerization methods, polymers having different structure are obtained, and their physical properties differ from each other.

Cyclic olefin-based addition polymers obtained by addition polymerization using homogeneous catalyst, unlike polymers obtained by ROMP or copolymerization with ethylene, have rigid and bulky ring structure in three-dimensions in every monomer unit of the main chain. Thus, the conformational properties of polymer chains differ from those of polymers prepared by ROMP or copolymerization with ethylene, and they are amorphous polymers having comparatively higher glass transition temperature. Particularly, norbornene-based polymers having comparatively large molecular weight obtained by addition polymerization using an organic metal compound as a catalyst are mostly polymers of non-polar alkyl norbornene or copolymers with olefin, or comprise tri-ethoxy silyl norbornene having weak polarity as monomers, and they have low dielectric constant and excellent electrical anisotropy (J. Appl. Polym. Sci. Vol 80, p 2328, 2001).
Introduction of substituent groups into a polymer consisting of hydrocarbon is useful for controlling chemical and physical properties of polymer. However, since free electron pair at such substituent group often reacts with activated catalyst site to act as a poison of a catalyst, it is not easy to introduce substituent group into a polymer. In case cyclic monomers having substituent groups are polymerized, the obtained polymer has low molecular weight. Generally, norbornene-based polymers are polymerized using transition organic metal catalyst, most of which show low activity for polymerizing monomers comprising polar groups such as ester or acetate group, and the produced polymer has a molecular weight of 10,000 or less (Risse et al., Macromolecules, 1996, Vol. 29, 2755-2763; Risse et al., Makromol. Chem. 1992, Vol. 193, 2915-2927; Sen et al., Organometallics 2001, Vol 20, 2802-2812, Goodall et al., U.S. Pat. No. 5,705,503).